The present invention relates generally to operational amplifier (opamp) circuits, and in particular to rail-to-rail complementary metal-oxide-silicon (CMOS) power opamp circuits.
Rail-to-rail power opamp circuits typically use a push-pull output structure with a PMOS pull-up transistor and an NMOS pull-down transistor. An example of such opamp can be found in "Large Swing Power Amplifier," by Kevin Brehmer and James Weiser, IEEE Journal of Solid State Circuits, Vol. SC-18, No. 6, pp. 624-629 (December 1983). In addition to an input amplifier stage, the Brehmer opamp employs two smaller error amplifiers for the push-pull output stage. The primary disadvantage of this opamp is its complexity and the resulting difficulty involved in stabilizing it. Each one of the error opamps requires its own feedback loop, while at the same time the larger amplifier, which includes the input amplifier and the two smaller error amplifiers, must be stabilized. This also limits the size of the capacitive load the opamp can drive in order to maintain sufficient phase margin. The article provides experimental data for capacitive loads up to 1000 pF only.
An improvement was offered by John Fisher in "A High Performance CMOS Power Amplifier," IEEE Journal of Solid States Circuits, Vol. SC-20, No. 6, pp. 1200-2105 (December 1985). Fisher's opamp combines the previous circuit by Brehmer and Wieser with a circuit that uses source-follower outputs. This makes the overall circuit easier to stabilize. However, the output is no longer rail-to-rail.
There is, therefore, a need for a rail-to-rail power opamp that can drive a large capacitive load, where the load capacitance is the dominant pole.